The present invention relates generally to magneto-optical data storage systems and, more particularly, to magneto-optical data storage systems having optical components positioned directly on the read/write heads.
Magneto-optical (MO) data storage systems provide storage of data on a rotating disk onto which a magneto-optical recording material has been deposited. The data is stored in the magneto-optical material as spatial variations of magnetic domains. During readout, the pattern of magnetic domains modulates the polarization of an incident laser beam, and a detection system converts the resulting optical signal into an electrical signal.
In one particular magneto-optical storage system, a magneto-optical head is positioned on a linear actuator that moves the head linearly along a radial direction of the disk. A magnetic coil creates a magnetic field having one of its components oriented in a direction perpendicular to the disk surface to produce a "vertical" magnetization vector in the disk. The vertical magnetization vector is recorded in the magneto-optical material by focusing a laser beam at a spot on the disk and heating the material above its Curie point. This is the temperature at which the magnetization in the material may be readily altered by an applied magnetic field. A current is then passed through the magnetic coil to orient the vertical magnetization vector in the material in either an up or a down direction to signify either a binary one or a binary zero bit value. The orientation of the magnetization vector remains after the laser beam is removed and the material cools. After a bit is recorded, it can be erased or overwritten by reheating the same spot above its Curie point and applying a magnetic field in the opposite direction.
The data recorded on the magneto-optical disk is retrieved using the magnetic Kerr effect. This is a phenomenon in which the magnetization of a spot on the disk causes a small rotation of the polarization of a laser beam incident that is on the spot. The magnitude of this Kerr rotation is determined by the material's Kerr coefficient, while the direction or sense of the rotation, whether clockwise or counter-clockwise, depends on the direction of the vertical magnetization vector of the spot. The direction of the rotation is measured by a differential detection scheme.
One major advantage of magneto-optical data storage systems is the higher real storage densities of these storage systems compared to magnetic data storage systems. However, the volumetric storage capacities of magneto-optical data storage systems have not kept pace with the volumetric storage capacities of magnetic data storage systems. One major factor limiting the volumetric storage capacity of an MO disk drive has been the relatively large size of the read/write head, which limits the spacing that can be achieved between the magneto-optical disks in a disk stack. The large size and mass of the head also limit tracking bandwidth, track density and the speed at which information can be accessed from the MO disk.
One approach for improving the volumetric storage capacities of magneto-optical data storage systems has involved the use of flying magneto-optical heads. Flying MO heads generate lift forces through aerodynamic interactions between the flying head and the rotating magneto-optical disk in the same way as a Winchester flying head in a magnetic disk storage device. These lift forces are opposed by equal and opposite spring forces applied by the suspension, which maintains the flying head at a predetermined flying height over the disk surface. Because this flying height is much less than the height of a fixed head from the disk, the use of flying heads allows the spacing between the magneto-optical disks to be reduced.
One particular magneto-optical data storage system employing a flying magneto-optical head utilizes an RF-modulated Fabry-Perot (FP) laser source and a single optical fiber for optically coupling the laser source to the flying head. The optical fiber directs the incident laser beam to a servo controlled mirror that directs the laser beam onto the rotating disk. The mirror also directs the reflected laser beam back to the single optical fiber after it is reflected from the disk. The reflected beam contains the rotated polarization information. The fiber directs the reflected laser beam to fixed optical components located remotely from the flying head for optically processing the polarization states of the rotated polarization components of the reflected laser beam. The resulting differential intensity beams are then applied to a differential detector. This design yields a smaller and lighter flying head for improved system data density and volumetric storage capacity. It also offers mechanical simplicity at a lower cost.
This single optical fiber head design requires that the polarization states of the rotated polarization components of the reflected laser beam be preserved through the entire optical path, including the single optical fiber. Therefore, the use of a polarization maintaining (PM) optical fiber is necessary. However, the birefringent nature of a polarization maintaining (PM) optical fiber combined with certain characteristics of the RF-modulated Fabry-Perot (FP) laser diode causes some undesirable results. Birefringence is a characteristic of many optical materials in which the index of refraction (a function of the velocity of light in the medium) depends on the direction of polarization of the light propagating in the material. Birefringence causes a phase shift between orthogonally polarized waves which is proportional to the difference in the refraction indices times the path length divided by the wavelength. The RF-modulated FP laser diode produces a relatively broad-spectrum, multi-wavelength incident light beam having wavelengths that fluctuate with time (mode competition). Because of the birefringent characteristics of the optical fiber, these multiple fluctuating wavelengths cause signal components in the return beam to have polarization states that also fluctuate. These fluctuations in polarization appear as noise in the differential detector, which limits the achievable data rate for a given signal level. These optical fibers also exhibit polarization mode coupling and leakage. As a laser beam travels along the fiber, such mode coupling and leakage causes some of a parallel polarization component to appear as a perpendicular polarization component, and vice versa. This polarization leakage also appears as noise at the differential detector.
Accordingly, there has been a need for an improved flying magneto-optical head that is compact and lightweight and has reduced sensitivity to laser mode competition and polarization mode coupling/leakage. The present invention fulfills this need.